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What Doctors Don't Tell You

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January 2020 (Vol. 11 Issue 4)

Fish oils
About the author: 

Fish oils image

Fish oils: Our various stories about fish oils and the link to stroke caused a great deal of interest - and concern - among our readers

Fish oils: Our various stories about fish oils and the link to stroke caused a great deal of interest - and concern - among our readers.

As the questions were coming in thick and fast, we thought it better to put all your concerns to one of the UK's leading nutritionists, Dr Damien Downing.

Here is his response (and numbers that appear in brackets refer to a reference at the end of his reply):

1. Do fish oils cause/increase stroke risk?

The last year or so has seen important evidence emerging on how necessary and beneficial fish oils can be in inflammatory diseases, including heart disease, and in neurodevelopmental problems, such as hyperactivity, autism etc. We should not be surprised that they are now attracting attacks from, presumably, Big Pharma. This seems to be a good example of such attacks.

The logic of this appears to go thus:

a) Mercury can increase risk of stroke (and other cardiovascular diseases):
The evidence on this is conflicting; the first of these studies (1 - on 1833 Finnish men) found an association, the second (2. -on 1462 Swedish women) did not. On balance, it is probably safest to assume that it might be true.

b) Fish & fish oils can be contaminated with mercury:
True, but not much; one study (3) found fish oil capsules to contain less than 0.2 mcg (microgrammes) of mercury per gram of omega-3, implying a maximum total mercury intake from an appropriate fish oil dose of perhaps 0.6 mcg per day. Now, look at these figures from WHO (4):

Dental amalgam Min: 3.0 mcg - Max: 17.0 mcg
Fish/seafood 2.3 mcg
Other food 0.3 mcg
Air + water negligible

So the total exposure from such supplements can only be about 20 per cent of the exposure from dental amalgam, and is probably much less - yet dental amalgams are still widespread and regarded as safe! In this context, the new UK government recommendation that pregnant women should avoid canned tuna is probably sensible, but much less important than avoiding dental amalgams.

c) So fish oil could increase stroke risk:
Well, not really. This study (5) found that men (1871 Finnish men) with a high DHA (the most important component of omega-3 oils) and a low mercury had about one-third the risk of cardiovascular disease of men with low DHA and high mercury. So fish oil, as has been repeatedly demonstrated, helps to prevent cardiovascular disease, including stroke, although the mercury contained in it may increase stroke risk - but not nearly enough to outweigh the benefit from the oil.

2. What about fish liver oils?

The same story, but also liver oils contain lots of vitamins A & D. We used to think that the vitamin D was dangerous in such doses, but current evidence disproves this. The vitamin A might be harmful to pregnant women and their fetuses (personally I doubt it), so I would recommend that no more than 1 fish liver oil could be substituted for a fish oil capsule in a daily regime.

3. How well are fatty acids absorbed from plant oils?

Not as well, it would seem, at least for omega-3 oils. Flax seed oil, the evidence suggests, is about 10 times less effective at raising omega-3 levels. So should we switch from fish oils to plant oils? My advice is no, but if you want to add a plant oil into the mix, that is fine.

4. How does diet and intestinal health affect absorption?

It's got to be important, though stomach acid and liver function are probably more important than bowel flora; by the age of 60, about half of us have reduced stomach acid and, without healthy bile production, fat absorption will be impaired.

5. Does increased fat/oil intake increase oxidative stress?
Yes, any oil will increase your requirements for antioxidants. Not by a huge amount though, and most such supplements contain some antioxidants such as vitamin E which, being fat-soluble, is the best choice. I always advise taking extra mixed antioxidants with the oils.

1. Salonen, Jukka T., M.D., M.P.H., et al. Intake of Mercury From Fish, Lipid Peroxidation, and the Risk of Myocardial Infarction and Coronary, Cardiovascular, and Any Death in Eastern Finnish Men. Circulation 1995; 91(3): 645-655.

2. Ahlqwist M, et al. Serum Mercury Concentration in Relation to Survival, Symptoms, and Diseases: Results From the Prospective Population Study of Women in Gothenburg, Sweden. Acta Odontol Scand, 1999;57:168-174.

3. Rissanen T, Voutilainen S, Nyyssonen K, et al. Fish Oil-Derived Fatty Acids, Docosahexaenoic Acid and Docosapentaenoic Acid, and the Risk of Acute Coronary Events: The Kuopio Ischaemic Heart Disease Risk Factor Study. Circulation 2000;102:2677-2679.

4. World Health Organization, Environmental Health Criteria 118: Inorganic Mercury, Geneva, 1991.

5. Burns CP, Halabi S, et al. Phase I Clinical Study of Fish Oil Fatty Acid Capsules for Patients With Cancer Cachexia: Cancer and Leukemia Group B Study 9473. Clin Cancer Res 1999;5:3942-3947.

Thank you so much, Damien. It may be hard for Damien to enter into personal correspondence with you over this (there are just so many of you now receiving E-news), so please send further questions via the E-news desk at If you'd like a consultation with Damien, he can be contacted via telephone 0845 166 2058, or email:

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